Light-emitting diodes (LEDs) are attractive replacement candidates for conventional light sources based on incandescent bulbs and fluorescent light tubes. LEDs have higher energy conversion efficiency than incandescent lights and substantially longer lifetimes than both incandescent and fluorescent light fixtures. In addition, LED-based light fixtures do not require the high voltages associated with fluorescent lights.
Unfortunately, LEDs have other problems that must be overcome before LED-based lighting units can replace conventional lighting units. For example, LEDs age over time such that the light output for a given current decreases with time. To compensate for this aging, feedback loops are often utilized in which a photodiode is incorporated with the LED to measure the light output from the LED. The signal from the photodiode is utilized by a servo loop to alter the average current through the LED such that the light output from the LED remains constant.
When a light source is constructed from multiple LEDs, the placement of the light sensor for the feedback loop becomes problematic. The individual LEDs are distributed over a finite area within the light source; hence, the light will only appear to have the desired color at locations that are at a distance from the LEDs that is large compared to the maximum distance between the LEDs so that the light is well mixed at the viewing location. Alternatively, a mixing cavity of some type is provided to mix the light such that light leaving the cavity is well mixed. The light sensor must be located at a point at which the light from the individual LEDs has been sufficiently mixed to provide the desired color. Hence, there can be a significant distance between the individual LEDs and the point at which the light is sampled to provide the signals for the servo loop even within a single light source. The problems are more severe in lighting systems that involve multiple light sources that target different physical areas for illumination, since there is no longer a single point at which light from all of the light sources is uniformly mixed.
In general, the photodetectors used to monitor the light from the LEDs are preferably located on the same die as the controller that executes the servo loop and the drivers used to power the individual LEDs. In light sources having multiple target areas, a separate controller chip must be used for each target area, which increases the cost of the light source. Further, the size of the die is much larger than the size of the photodiodes; hence, a significant area of the light path can be blocked by the controller die, which possesses significant design, as well as efficiency, problems in some applications.